SizeMgr::text_rules, line_height, handle_len; AdaptWidget::with_min_size_px/_em

[dhardy]

• Add Coord/Size/Offset::clamp
• Add SizeMgr::handle_len (for scroll bars and slider)
• Reserve: allow parametrization via nameable trait impl
• Add AdaptWidget::with_min_size_px, .._em methods
• Re-add SizeMgr::line_height
• Revise SizeMgr::text_bound: rename to text_rules, less affected by text class
• Implicit text labels in layout: disable wrapping. (Use explicit Label::new("...") to enable wrapping.)
• Accelerator keys: on key conflict, first binding takes priority.
• Several little fixes.